International Publication No. 2009/011374 describes an isolated DC-DC converter. The primary side of a transformer of this DC-DC converter is configured such that an LC filter circuit and a switching circuit are connected, in order, to an input power source. There is provided a current transformer between the filter circuit and the switching circuit and this current transformer detects a current flowing in the primary side. When the detected current is greater than or equal to a predetermined value, overcurrent protection control is performed.
When a step-like voltage is applied to the filter circuit, for example, at the time when a power source is turned on in the DC-DC converter described in the foregoing International Publication, the filter circuit's inductance causes a surge voltage greater than the input voltage. In the case where this surge voltage is applied to the switching circuit, FETs that constitute the switching circuit and that are connected in series may be damaged by the surge voltage. In addition, in the case where there are variations in the parasitic capacitances of the series-connected FETs, a capacitance ratio becomes out of balance. In this case, when the surge voltage is applied to the switching circuit, the surge voltage may be greater than the voltages that the FETs can resist. As a result, high-voltage FETs need to be used as switching elements, resulting in a high-cost.
A primary objective of the present invention is to provide a DC-DC converter that can prevent, at low cost, switching elements from being damaged by surge voltage.